1. Field of the Invention
This invention relates to a semiconductor laser driving circuit for driving a semiconductor laser used for recording and reproducing in an optical recording and reproducing apparatus.
2. Description of the Prior Art
FIG. 6 shows a conventional semiconductor laser driving circuit. When effecting using an optical recording and reproducing apparatus, an output beam of a semiconductor laser 1 is switched on and off in response to a recording signal. In particular, a current flow to the semiconductor laser 1 is switched on and off by means of a differential switching circuit composed of PNP transistors 2a and 2b. A current source 5 sets a peak value of a switching current applied to the semiconductor laser. A recording signal input 3 is supplied to the base of the PNP transistor 2a, and resistors 4a and 4b are connected to the base of the PNP type transistor 2b to set a threshold. The current flowing to the semiconductor laser is differentially switched in response to the recording signal output 3.
The semiconductor laser 1 shown in FIG. 6 has a cathode thereof connected to a metal case due to the structure of the semiconductor laser device or in order to perform heat radiation efficiently. The metal case of the semiconductor laser is usually grounded to a metallic part of an optical head to be equal in potential to the ground terminal of the driving circuit. In order to switch the semiconductor laser 1 whose cathode is grounded as above, the differential switching circuit is a current discharging type, and thus it is required to be composed of PNP type transistors.
With the structure as shown above, however, a switching operation having both a large magnitude of current back as 100 to 200 mA and a switching speed cannot be easily realized by means of PNP type transistor devices. This is because in the p-type semiconductor it is difficult to attain a switching characteristic responsive to a large magnitude of current at a high speed from the viewpoint of the device structure. As a result, with a P-type switching device, if a recording signal is inputted at a high speed of about 50 Mbps, the necessary high speed switching of the semiconductor laser is not possible even if a differential switching structure is employed.